The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In an automatic parking assistance maneuver, besides path planning for the vehicle, the controller needs to perform vehicle motion control along the planned path with necessary acceleration and deceleration controls. The acceleration is facilitated using engine torque control, and the deceleration is facilitated using brake control based on the desired vehicle target acceleration or deceleration level, and, inevitably, the gravitational force arising out of the road slope. In other words, the vehicle motion control during automatic parking maneuver requires knowledge of the three operating forces on the vehicle: The engine torque, the brake force and the slope of the road surface.
While information of engine torque and brake force can be obtained via respective controllers onboard the vehicle, slope of road surface is unavailable. Therefore, it is essential that information of the road slope be obtained for a satisfactory vehicle motion control during the automatic parking maneuver.
When the vehicle is equipped with a longitudinal chassis accelerometer, its signal may be applied for road slope estimation. In an ideal situation, when the vehicle is standing still on a slope, the longitudinal chassis accelerometer should provide the reading of slope in terms of an equivalent vehicle acceleration value on leveled ground. However, engineering issues may arise in various aspects when using a chassis longitudinal accelerometer:
First of all, the signal of a chassis accelerometer is naturally very noisy as it must be sensitive and responsive to the slightest motion of the vehicle, thus making the reading inevitably inaccurate; besides, the nature of noisy signal requires filtering which causes dynamic delay in the signal;
In addition, the vehicle is mostly in motion during the parking maneuver, and, therefore, dynamic delay in the filtered signal means delay of road-slope signal to the APA controller. This situation may not pose any problem when the road slope is fairly constant; however, challenge arises when the road slope is varying during the parking maneuver;
Moreover, while the chassis accelerometer is fixed in location and orientation with respect to vehicle chassis referenced to, say, the vehicle forward-moving direction, parking maneuvers require the vehicle to go in cycles of forward and reverse motion. As a result, the road slope, as perceived by the APA controller, will change sign even on the same spot when the transmission gear is changed from forward to reverse, and vice versa;
Furthermore, while the chassis accelerometer may provide accurate reading of road slope during its stand-still condition, the vehicle can hardly be operated in such condition during the automatic parking maneuver as it is constantly switching between forward and reverse gears with only very brief moment of standing still. The problem is made worse when the brief moment of standing still is compounded with the dynamic time delay of signal filtering that prevents the filter to reach the otherwise steady-state value of the road-slope information during the brief standing-still; and
Last but not the least, the biggest challenge of road slope estimation is attributed to vehicle chassis dynamics, especially vehicle pitch motion arising out of vehicle acceleration and deceleration. This problem is especially significant when the vehicle suspension has lower stiffness.
Therefore, it is the objective to design and develop a system and a method as an integral part of the system to provide accurate and timely information of road surface slope for satisfactory controls of vehicle motion during automatic parking maneuver, especially taking into account the factors of soft vehicle suspension and situation of varying road slope.